Merge topic 'def-generate-windows-only'

1ac042aa67 Do not generate rules for .def generation where not supported

Acked-by: Kitware Robot <kwrobot@kitware.com>
Merge-request: !1809

Source/cmGeneratorTarget.cxx

@@ -1999,8 +1999,13 @@ void cmGeneratorTarget::ComputeModuleDefinitionInfo(
   info.WindowsExportAllSymbols =
     this->Makefile->IsOn("CMAKE_SUPPORT_WINDOWS_EXPORT_ALL_SYMBOLS") &&
     this->GetPropertyAsBool("WINDOWS_EXPORT_ALL_SYMBOLS");
+#if defined(_WIN32) && defined(CMAKE_BUILD_WITH_CMAKE)
   info.DefFileGenerated =
     info.WindowsExportAllSymbols || info.Sources.size() > 1;
+#else
+  // Our __create_def helper is only available on Windows.
+  info.DefFileGenerated = false;
+#endif
   if (info.DefFileGenerated) {
     info.DefFile = this->ObjectDirectory /* has slash */ + "exports.def";
   } else if (!info.Sources.empty()) {